Can the system detect that the TRU is unlocked if the pilot hasn't selected reverse (e.g. because there's a separate mechanical lock that releases on ground even if the reversers haven't been selected)?
If not, it seems like the best chance to detect a fault (before it's too late) might be if the pilot selects reverse a second or two early and the TRU unlocks when it shouldn't. If you delay the alarm a fixed number of seconds, you're likely to also lose coverage for those critical couple of seconds before there's weight on the main gear.